fix(subdocument): make subdocument#isModified use parent document's isModified

Fix #8223

Author: vkarpov15

lib/types/subdocument.js

@@ -115,6 +115,16 @@ Subdocument.prototype.markModified = function(path) {
   }
 };
 
+Subdocument.prototype.isModified = function(paths, modifiedPaths) {
+  if (this.$parent && this.$basePath) {
+    paths = (Array.isArray(paths) ? paths : paths.split(' ')).
+      map(p => [this.$basePath, p].join('.'));
+    return this.$parent.isModified(paths, modifiedPaths);
+  }
+
+  return Document.prototype.isModified(paths, modifiedPaths);
+};
+
 /**
  * Marks a path as valid, removing existing validation errors.
  *